1. Field of the Invention
The present invention relates to an optical scanning system which forms an optical image on an object and scans the object by sweeping the optical image over the object.
2. Related Art Statement
There is known a laser printer including an optical scanning system wherein an image-forming lens is provided with a non-reflective coating for preventing a ghost beam from being reflected therefrom and thereby preventing a ghost image from being formed on an object by the ghost beam subsequently reflected by one of mirror surfaces of a polygon mirror.
In addition, U.S. Pat. Nos. 5,191,463, 4,745,416, and 4,586,782 disclose the art of preventing a ghost image from being formed on an image surface.
In the above-indicated conventional optical scanning system, however, since the non-reflective coating must be provided on the image-forming lens for preventing the ghost image from being formed by the light beam partly reflected from the lens, an increased number of working steps are needed for producing the lens. In addition, the ghost-image preventing techniques disclosed by the above-indicated patents suffer from the problems explained in detail later.
Moreover, U.S. Pat. No. 4,586,782 discloses an optical scanning system wherein the optical axis of an image-forming lens is inclined with respect to a deflection plane defined by a light beam incident to a polygon mirror and the light beam deflected toward the lens by the mirror surfaces of the polygon mirror. However, the center line of the mirror surfaces of the polygon mirror parallel to the top and bottom edge lines of the mirror surfaces is aligned with, and is contained by, the deflection plane. However, the width or thickness, W, of the mirror surfaces of the polygon mirror cannot be reduced to smaller than about 3 mm because, if the width W becomes smaller than 3 mm, the mirror surfaces cannot be formed with satisfactory accuracy and accordingly the amount of distortion of an optical image formed on an object becomes excessively large. Therefore, the angle of inclination of the optical axis of the image-forming lens cannot be reduced from a certain lower limit, and similarly the amount of distortion of the optical image cannot be reduced from a certain lower limit.